The present invention relates to an umbilical for use in the offshore production of hydrocarbons, and in particular to a power umbilical for use in deep water applications.
An umbilical consists of a group of one or more types of elongated active umbilical elements, such as electrical cables, optical fibre cables, steel pipes and/or hoses, cabled together for flexibility, over-sheathed and, when applicable, armoured for mechanical strength. Umbilicals are typically used for transmitting power, signals and fluids (for example for fluid injection, hydraulic power, gas release, etc.) to and from a subsea installation.
The umbilical cross-section is generally circular, the elongated elements being wound together either in a helical or in a S/Z pattern. In order to fill the interstitial voids between the various umbilical elements and obtain the desired configuration, filler components may be included within the voids.
ISO 13628-5 “Specification for Subsea Umbilicals” provides standards for the design and manufacture of such umbilicals.
Subsea umbilicals are installed at increasing water depths, commonly deeper than 1000 m. Such umbilicals have to be able to withstand severe loading conditions during their installation and their service life.
The main load bearing components in charge of withstanding the axial loads due to the weight and to the movements of the umbilical are steel pipes, steel rods, composite rods, or tensile armour layers.
The other elements, i.e. the electrical and optical cables, the thermoplastic hoses, the polymeric external sheath and the polymeric filler components, do not contribute significantly to the tensile strength of the umbilical.
Electrical cables used in subsea umbilicals fall into two distinct categories respectively known as signal cables and power cables.
Signal cables are used for transmitting signals and low power (<1 kW) subsea, such as to electrical devices on the seabed. Signal cables are generally rated at a voltage smaller than 3000V, and typically smaller than 1000V. Signal cables generally consist of small-section insulated conductors bundled together as pairs (2), quads (4) or, very rarely, any other number, the bundle then being over-sheathed.
Power cables are used for transmitting high electrical power (typically a few MW) subsea, such as to powerful subsea equipments such as pumps. Power cables are generally rated at a medium voltage comprised between 6 kV and 35 kV. A typical power cable is illustrated in the accompanying FIG. 1. Going from the inside layer to the outside layer, the power cable in FIG. 1 comprises a central copper conductor 2a, semi-conductor and electrical insulation layers 2b, a metallic foil screen 2c, and an external polymeric sheath 2d. The central conductor 2a generally has a stranded construction and a large cross-section, typically comprised between 50 mm2 and 400 mm2. Three phase power can be provided by three such cables bundled together within the umbilical structure.
An umbilical comprising at least one power cable is termed often a power umbilical. Thus, a power umbilical includes one or more electrical power cables, formed from one or more conductors, each conductor formed from one or more strands.
The conductors of these power cables within a subsea power umbilical are generally copper as specified in ISO 13628-5. They are not load bearing components because of the low yield strength and high specific gravity of copper. Moreover, these heavy copper conductors add considerable weight to an umbilical and have very poor load carrying capacity, thus limiting the sea depth that the umbilical can be deployed at. Unless protected, these electrical conductors may be damaged by excessive elongation or crushing, especially under severe conditions such as in deep water and/or dynamic umbilicals.